Lithium silicate glass ceramic and glass with ZrO2 content

ABSTRACT

Lithium silicate glass ceramics and glasses are described which can advantageously be applied to zirconium oxide ceramics in particular by pressing-on in the viscous state and form a solid bond with these.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S.application Ser. No. 13/079,861, filed Apr. 5, 2011, which claims thebenefit of European Patent Application Serial No. 10160222.5, filed Apr.16, 2010 and European Patent Application Serial No. 11162840.0 filedApr. 18, 2011 which are all hereby incorporated by reference in theirentirety.

TECHNICAL FIELD

The invention relates to lithium silicate glass ceramic and glass whichcomprise ZrO₂ and are suitable in particular for coating zirconium oxideceramic.

BACKGROUND OF THE INVENTION

Zirconium oxide ceramics are characterized by excellent biocompatibilityand outstanding mechanical properties, which is why in the past theyhave increasingly been used as a material for implants and prostheses,but also as framework materials for dental restorations. Ceramics basedon partially stabilized zirconium oxide are primarily used for this.

In many cases it is desirable to alter the surface of the zirconiumoxide ceramic by coating it with a different material. Specifically whenpreparing dental restorations based on zirconium oxide ceramic, such acoating is regularly used to give the restoration the desired visualproperties.

Glass ceramics have already been used in the past to coat or veneeroxide ceramics, such as zirconium oxide ceramics. These includefeldspar-based ceramics or fluoroapatite glass ceramics.

Lithium disilicate glass ceramics are also known which, because of theirhigh translucency and very good mechanical properties, are used inparticular in the dental field and primarily for preparing dental crownsand small bridges.

EP 1 505 041 and corresponding U.S. Pat. No. 7,316,740, the entiretywhich is hereby incorporated by reference, describe lithium silicateglass ceramics which can additionally contain 0 to 2 wt.-% ZrO₂. Theseare processed into the desired dental restorations in particular in theform of lithium metasilicate glass ceramics by means of CAD/CAM methods,wherein a subsequent heat treatment effects the conversion of themetasilicate phase to the high-strength disilicate phase. The glassceramics can also be used for pressing over ceramic restorations.

EP 1 688 398 and corresponding U.S. Pat. No. 7,452,836, the entiretywhich is hereby incorporated by reference, describe similar lithiumsilicate glass ceramics which are substantially free of ZnO and inaddition to other components can contain 0 to 4 wt.-% ZrO₂. To achievehigh strengths, however, small quantities of from 0 to 2 wt.-% ZrO₂ arepreferred. These glass ceramics also serve in particular to preparedental restorations after mechanical processing by means of CAD/CAM.

However, these lithium silicate glass ceramics known from the state ofthe art have the disadvantage that they are not suitable for coatingzirconium oxide ceramic in particular by means of a pressing-on in theviscous state, since after the pressing-on in the viscous flow processcracks and flaws form in the glass ceramic. Thus, such a composite doesnot have the mechanical properties that are indispensable specificallyfor use as dental restoration material.

Glass ceramics with lithium disilicate as main crystal phase which areto be suitable for veneering dental restorations comprisingyttrium-stabilized zirconium dioxide are also known from WO 2008/106958and corresponding U.S. Published Application No. 2011030423, theentirety which is hereby incorporated by reference. However, these glassceramics contain quantities of only up to 6.0 wt.-% of ZrO₂ andsubstantial quantities of Na₂O. The ZrO₂ present acts merely as astandard nucleating agent together with an optionally present furthernucleating agent such as TiO₂ in order to effect the formation of thedesired lithium disilicate crystal phase.

SUMMARY OF THE INVENTION

In view of the above-described disadvantages of the already known glassceramics, the object of the invention is to provide a glass ceramicwhich can be coated onto a zirconium oxide ceramic in particular bypressing it on in the viscous state and in the process forms a coatingsubstantially free of cracks and flaws. Moreover, the glass ceramicshould be capable of forming a solid bond with the zirconium oxideceramic to be coated, and it should have visual and mechanicalproperties enabling it to be used in particular as a coating materialfor dental restorations but also as a material for preparing dentalrestorations.

This object is achieved by the lithium silicate glass ceramic accordingto any one of the claims, incorporated herein. Also a subject of theinvention are the lithium silicate glass with nuclei, the startingglass, the process for preparing the glass ceramic and the glass withnuclei, the use, the process for coating a zirconium oxide ceramic andthe coated zirconium oxide ceramic.

The lithium silicate glass ceramic according to the invention ischaracterized in that it comprises at least 6.1 wt.-% ZrO₂ and inparticular at least 6.5, preferably at least 7.0, more preferably atleast 8.0 and particularly preferably at least 10.0 wt.-% ZrO₂.

In a further preferred embodiment, the glass ceramic comprises inparticular 6.1 to 20.0 wt.-%, preferably 8.0 to 20.0 wt.-%, particularlypreferably 8.0 to 18.0 wt.-% and quite particularly preferably 10.0 to16.0 wt.-% ZrO₂.

Further, a glass ceramic which comprises 55.0 to 71.0, preferably 60.0to 71.0 and in particular 60 to 69 wt.-% SiO₂ is preferred.

In addition, a glass ceramic which comprises 9.0 to 17.0 and inparticular 11 to 15 wt.-% Li₂O is preferred.

Furthermore, it has proven particularly preferable if the glass ceramiccomprises 0.5 to 12.0 and in particular 2.5 to 7.0 wt.-% nucleatingagents. Preferred nucleating agents are selected from P₂O₅, TiO₂, Nb₂O₅,metals, e.g. Pt, Pd, Au and Ag, or mixtures thereof. Particularlypreferably, the glass ceramic comprises P₂O₅ as nucleating agent.Surprisingly, P₂O₅ as nucleating agent in particular effects theformation of desired lithium disilicate crystals while largelypreventing the formation of ZrO₂-containing crystal phases which couldcause a substantial deterioration in translucency. Through its use theformation of other undesired secondary crystal phases is apparently alsolargely prevented.

The glass ceramic according to the invention preferably comprises afurther alkali metal oxide in a quantity of from 1.0 to 7.0, preferably2.0 to 7.0 and particularly preferably 2.0 to 5.0 wt.-%. The term“further alkali metal oxide” denotes alkali metal oxide with theexception of Li₂O. The further alkali metal oxide is in particular K₂O,Cs₂O and/or Rb₂O and is particularly preferably K₂O. It is assumed thatthe use of K₂O contributes to the strengthening of the glass networkcompared with the Na₂O used in conventional glass ceramics. It ispreferred that the glass ceramic comprises less than 2.0, in particularless than 1.0, preferably less than 0.5 and particularly preferablyessentially no Na₂O.

It is further preferred that the glass ceramic comprises up to 5.0 wt.-%alkaline earth metal oxide, wherein the alkaline earth metal oxide is inparticular CaO, BaO, MgO, SrO or a mixture thereof.

Further, a glass ceramic which comprises 0.2 to 10.0, in particular 2.5to 7.0 and preferably 2.5 to 3.5 wt.-% oxide of trivalent elements ispreferred, wherein this oxide is selected in particular from Al₂O₃,Y₂O₃, La₂O₃, Bi₂O₃ and mixtures thereof, and preferably is Al₂O₃.

A glass ceramic which comprises at least one and preferably all of thefollowing components is particularly preferred:

Component wt.-% SiO₂ 55.0 to 71.0 Li₂O  9.0 to 17.0 K₂O  1.0 to 7.0, inparticular 2.0 to 5.0 Al₂O₃  0.5 to 5.0, in particular 2.5 to 3.5 P₂O₅ 0.5 to 12.0, in particular 2.5 to 7.0 ZrO₂  6.1 to 20.0, in particular8.0 to 20.0.

The glass ceramic according to the invention can moreover also compriseadditional components which are selected in particular from furtheroxides of tetravalent elements, further oxides of pentavalent elements,oxides of hexavalent elements, melt accelerators, colourants andfluorescent agents.

The term “further oxides of tetravalent elements” denotes oxides oftetravalent elements with the exception of SiO₂ and ZrO₂. Examples offurther oxides of tetravalent elements are SnO₂ and GeO₂.

The term “further oxides of pentavalent elements” denotes oxides ofpentavalent elements with the exception of P₂O₅. An example of a furtheroxide of pentavalent elements is Bi₂O₅.

Examples of oxides of hexavalent elements are WO₃ and MoO₃.

A glass ceramic which comprises at least one further oxide oftetravalent elements, one further oxide of pentavalent elements or oneoxide of hexavalent elements is preferred.

Examples of melt accelerators are fluorides.

Examples of colourants and fluorescent agents are oxides of d-andf-elements, such as e.g. the oxides of Ti, Sc, Mn, Fe, Ag, Ta, W, Ce,Pr, Nd, Tb, Er and Yb.

The term “main crystal phase” used below denotes the crystal phase whichhas the highest proportion by volume compared with other crystal phases.

The glass ceramic according to the invention preferably compriseslithium metasilicate as main crystal phase. In particular the glassceramic comprises more than 10 vol.-%, preferably more than 20 vol.-%and particularly preferably more than 30 vol.-% of lithium metasilicatecrystals, relative to the total glass ceramic.

In a further preferred embodiment, the glass ceramic comprises lithiumdisilicate as main crystal phase. In particular the glass ceramiccomprises more than 10 vol.-%, preferably more than 20 vol.-% andparticularly preferably more than 30 vol.-% of lithium disilicatecrystals, relative to the total glass ceramic.

BRIEF DESCRIPTION OF THE FIGURES

Further advantages, details and features emerge from the followingdescription of embodiments of the invention on the basis of thedrawings, in which:

FIG. 1 shows the result of the differential scanning calorimetry (DSC)of a pounded glass cylinder according to Example 29.

FIG. 2 shows, using high-temperature X-ray diffraction (HT-XRD) of aglass cylinder, the dependence of the formation of lithium metasilicate(Li2SiO3) and lithium disilicate (Li2Si2O5) from the temperature.

FIG. 3 shows a scanning electron microscopy (SEM) photograph of acrystallized cylinder of Example 29 which has been polished and etchedfor 30 s with HF vapour.

FIG. 4 shows a scanning electron microscopy (SEM) photograph of the bondbetween a lithium disilicate glass ceramic according to Example 5 and azirconium oxide ceramic after etching with 40% HF vapour.

DETAILED DESCRIPTION

The lithium disilicate glass ceramic according to the invention showsparticularly good mechanical properties and can be produced by heattreatment of the lithium metasilicate glass ceramic according to theinvention.

It was surprisingly shown that, despite its high ZrO₂ content, thelithium disilicate glass ceramic according to the invention hasadvantageous mechanical parameters, such as high fracture toughnessvalues, and can be applied to zirconium oxide ceramic in the viscousstate by sintering-on or in particular pressing-on, without resultantstresses in the glass ceramic which manifest themselves in cracks orflaws. It is particularly surprising that these very good mechanicalproperties are achieved although the structure of the glass ceramic haslithium disilicate crystals which are not normally cross-linked to oneanother. On the other hand, such a cross-linking occurs with the knownlithium disilicate glass ceramics and it is regarded as a key reason fortheir high strengths. It is currently assumed that the ZrO₂ in the glassceramic according to the invention, unlike in known products, does notserve as a nucleating agent for other crystal phases, but ratherstrengthens the glass network via Zr—O polyhedra embedded therein. Thesepolyhedra can be [ZrO_(6/2)]²⁻ or [ZrO_(8/2)]⁴⁻ structural units whichfunction as network formers or network modifiers.

It is also surprising that, despite its high ZrO₂ content, the lithiumdisilicate glass ceramic according to the invention has a hightranslucency and that no amorphous-amorphous phase separation occurs andthat it can thus be used for the aesthetically pleasing coating of inparticular dental restorations based on zirconium oxide ceramic.

The lithium disilicate crystals present in the lithium disilicate glassceramic according to the invention have in particular the form of smallplates. It is assumed that this special morphology allows the crack-freematerial bond with zirconium oxide ceramics. The build-up of criticalstresses in the material bond during the thermal cooling phase seems tobe less strongly pronounced in the small-plate shaped crystal form thanin lithium disilicate glass ceramics with elongated or needle-shapedcrystals. In addition, a good fracture toughness, expressed by theK_(IC) value, is achieved with the small-plate shaped crystalmorphology.

The lithium disilicate glass ceramic according to the invention has inparticular a fracture toughness, measured as K_(IC) value, of at least1.5 MPa·m^(0.5) and in particular more than 1.8 MPa·m^(0.5).Furthermore, it has a high biaxial fracture toughness of preferably from200 to 500 MPa. Moreover, it shows a high chemical resistance which wasdetermined by mass loss after storage in acetic acid. The chemicalresistance is in particular less than 60 μg/cm². Finally, it has alinear coefficient of thermal expansion of in particular less than10.3×10⁻⁶K⁻¹ m/m, measured in the range of from 100 to 500° C., which isthus as a rule smaller than that of the zirconium oxide ceramic to becoated.

The invention also relates to a lithium silicate glass with nuclei thatare suitable for forming lithium metasilicate and/or lithium disilicatecrystals, wherein the glass comprises the components of theabove-described glass ceramics according to the invention. This glassthus comprises at least 6.1 wt.-% ZrO₂. With regard to preferredembodiments of this glass, reference is made to the above-describedpreferred embodiments of the glass ceramics according to the invention.

The glass with nuclei according to the invention can be produced by heattreatment of a correspondingly composed starting glass, which forms afurther aspect of the present invention. By a further heat treatment thelithium metasilicate glass ceramic according to the invention can thenbe formed, which in turn can be converted into the lithium disilicateglass ceramic according to the invention by further heat treatment. Thestarting glass, the glass with nuclei and the lithium metasilicate glassceramic can consequently be seen as precursors for the production of thehigh-strength lithium disilicate glass ceramic.

The glass ceramic according to the invention and the glass according tothe invention are present in particular in the form of powders orblanks, as they can easily be further processed in these forms. Theycan, however, also be present in the form of dental restorations, suchas inlays, onlays, crowns, veneers, or abutments.

The invention also relates to a process for preparing the glass ceramicaccording to the invention and the glass with nuclei according to theinvention, in which a starting glass with the components of the glassceramic or the glass is subjected to at least one heat treatment in therange of from 450 to 950° C.

The starting glass according to the invention therefore comprises atleast 6.1 wt.-% ZrO₂. In addition, it preferably also comprises suitablequantities of SiO₂ and Li₂O in order to make the formation of a lithiumsilicate glass ceramic possible. Furthermore, the starting glass canalso comprise further components, such as are given above for thelithium silicate glass ceramic according to the invention. Thoseembodiments are preferred which are also stated as preferred for theglass ceramic.

To prepare the starting glass, the procedure is in particular that amixture of suitable starting materials, such as e.g. carbonates, oxides,phosphates and fluorides, is melted at temperatures of in particularfrom 1300 to 1600° C. for 2 to 10 h. To achieve a particularly highhomogeneity, the obtained glass melt is poured into water in order toform a glass granulate, and the obtained granulate is then melted again.

The melt can then be poured into moulds to produce blanks of thestarting glass, so-called solid glass blanks or monolithic blanks.

It is also possible to pour the melt into water again in order toprepare a granulate. This granulate can then be pressed, after grindingand optionally adding of further components, such as colourants andfluorescent agents, to form a blank, a so-called powder green compact.

Finally, the starting glass can also be processed to form a powder aftergranulation.

The starting glass is then subjected, e.g. in the form of a solid glassblank, a powder green compact or in the form of a powder, to at leastone heat treatment in the range of from 450 to 950° C. It is preferredthat a first heat treatment is initially carried out at a temperature inthe range of from 500 to 600° C. to prepare a glass according to theinvention with nuclei which are suitable for forming lithiummetasilicate and/or lithium disilicate crystals. This glass can thenpreferably be subjected to at least one further temperature treatment ata higher temperature and in particular more than 570° C. to effectcrystallization of lithium metasilicate or lithium disilicate.

The at least one heat treatment carried out in the process according tothe invention can also take place during the pressing or sintering ofthe glass according to the invention or the glass ceramic according tothe invention onto the selected zirconium oxide ceramic.

Dental restorations, such as inlays, onlays, crowns, veneers orabutments, can be prepared from the glass ceramic according to theinvention and the glass according to the invention. The inventiontherefore also relates to their use for the preparation of dentalrestorations. In this connection it is preferred that the glass ceramicor the glass are shaped to the desired dental restoration by means ofpressing or machining. The pressing is usually conducted at elevatedpressure and elevated temperature. For the pressing in particular thestarting glass according to the invention and preferably the glass withnuclei according to the invention, the lithium metasilicate glassceramic according to the invention and the lithium disilicate glassceramic according to the invention can be used in a suitable manner,e.g. in the form of blanks. The machining is usually carried in aCAD/CAM process and the machining preferably uses the lithiummetasilicate and lithium disilicate glass ceramic according to theinvention, in particular in the form of suitable blanks. Afterpreparation of the dental restoration of the desired shape by pressingor machining it is possible to heat treat the restoration to convertprecursors, such as the starting glass, the glass with nuclei or lithiummetasilicate glass ceramic, into lithium disilicate glass ceramic.

The glass ceramic according to the invention and the glass according tothe invention are, however, suitable in particular for coating zirconiumoxide ceramics. The invention is therefore also directed towards the useof the glass according to the invention or the glass ceramic accordingto the invention for coating zirconium oxide ceramics.

The invention also relates to a process for coating zirconium oxideceramic, in which the glass ceramic according to the invention or theglass according to the invention is applied to the zirconium oxideceramic and is subjected to increased temperature.

This can take place in particular by sintering-on and preferably bypressing-on. With the sintering-on, the glass ceramic or the glass isapplied to the ceramic to be coated in the usual way, e.g. as powder,and then sintered at increased temperature. With the preferredpressing-on, the glass ceramic according to the invention or the glassaccording to the invention is pressed on, e.g. in the form of powdergreen compacts or monolithic blanks, at an increased temperature of e.g.from 700 to 1200° C., applying pressure, e.g. 2 to 10 bar. The methodsdescribed in EP 231 773 and the press furnace disclosed there can beused in particular for this. A suitable furnace is e.g. the Programat EP5000 from Ivoclar Vivadent AG, Liechtenstein.

It is preferred that, after conclusion of the coating process, the glassceramic according to the invention is present with lithium disilicate asmain crystal phase, as it has particularly good properties. It issurprisingly shown that the glass ceramic according to the invention haspractically no flaws and cracks once it has been coated onto thezirconium oxide ceramic, and a solid bond between glass ceramic andceramic is achieved.

It is preferred that the zirconium oxide ceramic comprises at least oneoxide of Ce, Y, Sr, Ca or Mg for stabilizing the tetragonal phase. Thezirconium oxide ceramic can also be present in the form of a compositewith other inorganic components.

The zirconium oxide ceramic coated with the glass ceramic according tothe invention or the glass according to the invention forms a furthersubject of the invention.

In view of the above-described properties of the glass ceramic accordingto the invention and the glass according to the invention as itsprecursor, they are also suitable in particular for use in dentistry. Asubject of the invention is therefore also the use of the glass ceramicaccording to the invention or the glass according to the invention as adental material and in particular for preparing dental restorations oras a coating material for dental restorations, such as crowns andbridges.

It is surprising that no cracks in the glass ceramic occur in the bondbetween the lithium disilicate glass ceramic according to the inventionand zirconium oxide ceramic. It is presumed that in particular thespecial small-plate shaped morphology of the lithium disilicate crystalsis of importance for this. The build-up of critical stress in thematerial bond during the thermal cooling phase seems to be less stronglypronounced in the small-plate shaped crystal form than in lithiumdisilicate glass ceramics with elongated or needle-shaped crystals. Inaddition a good fracture toughness of up to 2.1 MPa·m^(0.5) is achievedin particular with the small-plate shaped crystal morphology, although adirect cross-linking of the lithium disilicate crystals is essentiallynot to be seen in the structure. The coated zirconium oxide ceramicaccording to the invention is thus a strong compound betweenhigh-strength and high-toughness zirconium oxide ceramic on the one handand tough glass ceramic on the other, which is why this compound canabsorb high loads in the chewing cycle. The glass ceramic according tothe invention can thus advantageously also be used directly in thecoating of long-span bridges with more than three members based onzirconium oxide ceramic.

Finally, the glasses and glass ceramics according to the invention canalso be mixed with other glasses and glass ceramics to give dentalmaterials having desirably adjusted properties. Therefore, a glass orglass ceramic comprising the glass according to the invention or theglass ceramic according to the invention represents a further embodimentof the invention. The glass according to the invention or the glassceramic according to the invention can therefore in particular be usedas main component of an inorganic-inorganic composite or can be used incombination with a multitude of other glasses and/or glass ceramics.These composites or combinations are preferably used as dentalmaterials. It is particularly preferred to use the composites andcombinations in the form of sintered blanks. Examples of other glassesand glass ceramics for producing inorganic-inorganic composites andmixtures are disclosed in DE 43 14 817, corresponding U.S. Pat. No.5,432,130, DE 44 23 793, corresponding U.S. Pat. No. 5,698,019, DE 44 28839, corresponding U.S. Pat. No. 5,618,763, DE 196 47 739, correspondingU.S. Pat. Nos. 6,342,458, 5,968,856, and 6,514,893, DE 197 25 552 and DE100 31 431, all of which are hereby incorporated by reference. Theseglasses and glass ceramics belong to the silicate, borate, phosphate oraluminosilicate group. Preferred glasses and glass ceramics are of theSiO₂—Al₂O₂—K₂O type (with cubic or tetragonal leucite crystals),SiO₂—B₂O₂—Na₂O type, alkali-silicate type, alkali-zinc-silicate type,silico-phosphate type and/or SiO₂—ZrO₂ type. By mixing such glassesand/or glass ceramics with the glasses and/or glass ceramics accordingto the invention it is for example possible to adjust the thermalcoefficient of expansion in the desired manner in a broad range of 6 to20*10⁻⁶*1/K. The invention is described in further detail below withreference to examples.

EXAMPLES Examples 1 to 28 Composition and Crystal Phases

A total of 28 glasses and glass ceramics according to the invention withthe composition given in tables I to IV were prepared by meltingcorresponding starting glasses followed by heat treatment for controllednucleation and crystallization.

For this, the starting glasses were firstly melted in a 100 to 200 gscale from usual raw materials at 1400 to 1500° C. and transformed intoglass frits by pouring them into water. These glass frits were thenmelted a second time at 1450 to 1550° C. for 1 to 3 h for thehomogenization. The obtained glass melts were poured into pre-heatedmoulds to produce glass monoliths. These glass monoliths weretransformed into glasses and glass ceramics according to the inventionby thermal treatment.

The applied thermal treatment for the controlled nucleation andcontrolled crystallization is given in table V for selected examples.The first heat treatment in the range of from 500 to 560° C. usually ledto the formation of lithium silicate glasses with nuclei for lithiummetasilicate or lithium disilicate crystals, the second heat treatmentat 650 to 710° C. to the formation of lithium metasilicate glassceramics and the third heat treatment in the range of from 800 to 920°C. to the formation of lithium disilicate glass ceramics.

In some examples, a second non-isothermal heat treatment withsimultaneous analysis of the formed crystal phases was carried out atthe respectively given temperature by high-temperature X-ray diffraction(HT-XRD) after a first heat treatment.

The crystal phases obtained after conclusion of all the heat treatmentsare also listed in table V. Surprisingly, glass ceramics with lithiumdisilicate as main crystal phase were always obtained. Examples 5 and 6were additionally repeated, by carrying out only the first and secondheat treatment. In this way, glass ceramics with lithium metasilicate asmain crystal phase were produced.

Despite the high ZrO₂ content of up to 20 wt.-%, only in example 8 ZrO₂crystallized as secondary crystal phase.

Example 29 Glass and Glass Ceramic Blanks

A glass with the composition according to example 5 was prepared bymixing corresponding raw materials in the form of oxides and carbonatesfor 30 min in a Turbula mixer and then melting the mixture at 1450° C.for 120 min in a platinum crucible. The melt was poured into water inorder to obtain a finely divided glass granulate. This glass granulatematerial was melted again at 1530° C. for 150 min in order to obtain aglass melt with particularly high homogeneity. The temperature wasreduced to 1500° C. for 30 min and cylindrical glass blanks with adiameter of 12.5 mm were then poured into pre-heated, separable steelmoulds or graphite moulds. The obtained glass cylinders were thenstress-relieved at 550° C. A glass with nuclei for lithium metasilicateor lithium disilicate crystals was obtained.

FIG. 1 shows the result of the differential thermal analysis (DSC) of apounded glass cylinder.

FIG. 2 shows, using high-temperature X-ray diffraction (HT-XRD) of aglass cylinder, the dependence of the formation of lithium metasilicate(Li2SiO3) and lithium disilicate (Li2Si2O5) from the temperature.

The glass cylinders were then subjected to a first crystallization at680 to 700° C. for 20 min. The heating rate was 15° C. per minute. Theglass cylinders were then subjected to a second crystallization at 850to 880° C. for 30 min. After this treatment, the crystal phase analysisshowed a glass ceramic according to the invention with lithiumdisilicate as main crystal phase as well as small proportions of lithiummetasilicate and lithium phosphate as secondary phases.

FIG. 3 shows a scanning electron microscopy (SEM) photograph of acrystallized cylinder which has been polished and etched for 30 s withHF vapour.

The crystallized cylinders were moreover further processed byhot-pressing at a pressing temperature of 910° C. using an EP600 pressfurnace, Ivoclar Vivadent AG, to form testpieces. The properties ofthese testpieces were as follows:

-   Colour: white, translucent without fluorescence-   Solubility: 24 μg/cm² (according to ISO 6872 of Sep. 1, 2008)-   Biaxial strength: 420 MPa (according to ISO 6872 of Sep. 1, 2008)-   Fracture toughness: 2.0 MPam^(0.5) (determined as K_(IC) value using    the SEVNB method according to ISO 6872 of Sep. 1, 2008)-   Coefficient of thermal expansion: 9.9*10⁻⁶*1/K (in the range 100 to    500° C.)

Example 30 Hot-Pressing onto Zirconium Oxide Ceramic

The lithium disilicate glass ceramic according to example 5 was pressedby hot-pressing at 920° C. onto zirconium oxide ceramic of the type 3Y-TZP, obtainable from Tosoh, in a Programat EP 5000 combined press andfiring furnace from Ivoclar Vivadent AG, Liechtenstein. After conclusionof the coating process, a defect-free join resulted.

FIG. 4 shows a scanning electron microscopy (SEM) photograph of thisbond after etching with 40% HF vapour.

Example 31 Hot-Pressing onto Small Tooth Caps and Bridge Frameworks

Single small tooth caps and four-element bridge frameworks of denselysintered zirconium oxide (e.max ZirCAD, Ivoclar Vivadent AG) were madeup into anatomically shaped restorations with a plastic that can beburned out (PMMA). Both the bridge frameworks and the plastic parts weremanufactured by CAD/CAM processes, whereby a reproducible geometry andlayer thickness were able to be achieved. The restorations were investedin dental investment compound (IPS PressVest Speed, Ivoclar VivadentAG), the plastic was burned out and the crystallized cylinders accordingto example 29 were pressed directly onto the frameworks at a temperatureof 910° C. No intermediate layer (liner) was applied to the zirconiumoxide.

After the complete cooling, the articles were devested with asandblaster, wherein no special precaution was necessary, because of thehigh strength of the coated-on glass ceramic. The articles wereseparated from the compression channels, reworked dry with a diamondgrinder, and then treated for 20 min with IPS INVEX Liquid (IvoclarVivadent AG) under ultrasound, in order to loosen any remaining residuesof investment compound, which were then sandblasted with Al₂O₃ sand witha grain size of 100 μm at 1-2 bar pressure.

The surface was cleaned with hot steam and glazed twice with IPS e.maxCeram Glaze (Ivoclar Vivadent AG) at 770° C., whereby an attractivegloss was produced. No special cooling (relaxation cooling) was appliedduring the glazing firing. The thus-prepared restorations, i.e. crownsand bridges, were aesthetically pleasing and defect-free. They showed nocracks, bubbles or raised areas. After sawing, an excellent bond betweenthe coated-on lithium disilicate glass ceramic according to theinvention and the zirconium oxide was recognizable by means of SEM inthe cross-section.

In each case 8 crowns and 8 bridges were subjected to thermocycling offrom 5 to 55° C. in a chewing machine (Willitec) with 300,000 cycleswhile stored in water. The test strength was 30, 60 or 90 N during every100,000 cycles. The load was applied with a frequency of 0.8 Hz. Therewere no chippings at all in the veneer structure.

Example 32 Glass and Glass Ceramic Blanks

Example 29 was repeated with the difference that a glass with thecomposition according to example 23 was used as a starting material. Theobtained crystallized cylinders were further processed into testpiecesby hot-pressing at a temperature of 905° C. The properties of thesetestpieces were as follows:

-   Colour: tooth coloured, translucent with tooth-like fluorescence-   Solubility: 30 μg/cm² (according to ISO 6872 of Sep. 1, 2008)-   Biaxial strength: 405 MPa (according to ISO 6872 of Sep. 1, 2008)-   Coefficient of thermal expansion: 9.9*10⁻⁶*1/K (in the range 100 to    500° C.)

Example 33 Hot-Pressing onto Small Tooth Caps and Bridge Frameworks

Example 31 was repeated with the difference that the crystallizedcylinders according to example 32 were used. After up to four finalglazing firings, crowns and bridges were obtained which again showed nocracks, bubbles or raised areas.

Example 34 Glass Ceramic Blank (Powder Green Compact)

Analogously to examples 1 to 28, a starting glass with the compositionaccording to example 24 was melted twice. However, the glass was thennot poured into steel moulds, but quenched in water in order to obtain afinely divided glass granulate. The glass granulate material wasthermally treated at 550° C. for 20 min and then at 680° C. for 20 minin order to effect the nucleation and the first crystallization. Thethus pre-treated granulate was dry-ground to an average grain size of 20μm and mixed with 0.1 wt.-% of a ceramic colouring pigment. The mixturewas moistened with some water and pressed to form a powder green compactat a pressing pressure of 20 MPa. The powder green compact was sinteredat 850° C. for 30 min. The crystal phase analysis of the sintered blankshowed lithium disilicate as main crystal phase as well as in each casesmall proportions of lithium metasilicate and lithium phosphate assecondary phases.

The sintered blanks were further processed into testpieces byhot-pressing at 905° C. using the EP600 press furnace (Ivoclar VivadentAG). The properties of the testpieces were as follows:

-   Colour: tooth coloured, translucent and tooth-like fluorescence-   Biaxial strength: 302 MPa (according to ISO 6872 of Sep. 1, 2008)

Example 35 Hot-Pressing onto Small Tooth Caps

Example 31 was repeated with the difference that the sintered blanksaccording to example 34 were used to compress over small tooth caps.After two final glazing firings, crowns were obtained which again showedno cracks, bubbles or raised areas.

Example 36 Sintering onto Small Tooth Caps

Analogously to example 34, glass ceramic powder of the compositionaccording to example 24 coloured with 0.1 wt.-% pigment was prepared.However, this time, no powder blanks were pressed. The powder wasblended with a modelling liquid (e.max Ceram Build Up Liquid, IvoclarVivadent AG) and the mixture was applied to a prefabricated zirconiumoxide single small tooth cap in order to model an occlusal morphology.The coated-on mixture was then sintered in a dental furnace (P500,Ivoclar Vivadent AG) at a holding temperature of 850° C. and with aresidence time of 2 min.

After the sintering-on, the crowns were trimmed with diamond grindersand coated a second time. Two further glazing firings then took place ata temperature of 770° C. Aesthetically high-quality tooth-colouredcrowns with natural-looking fluorescence and opalescence resulted. Thesealso showed no cracks, bubbles or raised areas.

Example 37 Direct Preparation of Dental Restorations by Hot-Pressing orMachining (CAD/CAM)

(A) Blanks of Glass with Nuclei

First of all glasses having the composition according to examples 3, 4and 5 were prepared by mixing corresponding raw materials in the form ofoxides and carbonates for 30 min in a Turbola mixer and then melting themixture at 1450° C. for 120 min in a platinum crucible. The melts werepoured into water in order to obtain finely divided glass granulates.These glass granulates were melted again at 1530° C. for 150 min inorder to obtain glass melts with particularly high homogeneity. Thetemperature was reduced to 1500° C. for 30 min and subsequently a)rectangular glass blanks (12.5 mm×14 mm×40 mm) and b) cylindrical glassblanks with a diameter of 12.5 mm were then poured into pre-heated,separable steel moulds or graphite moulds. The obtained rectangularglass blocks or glass cylinders were then heat-treated in the range of500 to 560° C. depending on the composition to produce nuclei forlithium metasilicate and/or lithium disilicate crystals and tostress-relieve the glass.

The obtained blanks with nuclei were processed according to thefollowing alternatives to restorations.

(B) Hot-Pressing of Glass with Nuclei, Lithium Metasilicate or LithiumDisilicate Glass Ceramic

i) The glass cylinders with nuclei (A) were processed by hot-pressing ata temperature of 900-950° C. by means of a pressing furnace EP600,Ivoclar Vivadent AG, to give dental restorations, e.g. inlays, onlays,veneers, partial crowns and crowns.

ii) The glass cylinders with nuclei (A) were subjected to a firstcrystallization at 650 to 750° C. for 20 minutes. The heating-up ratewas 15° C. per minute. After this first crystallisation lithiummetasilicate was detected as main crystalline phase. Throughhot-pressing of the lithium metasilicate glass cylinders at a pressingtemperature of 900-950° C. using a pressing furnace EP600, IvoclarVivadent AG, it was possible to produce dental restorations, e.g.inlays, onlays, veneers, partial crowns and crowns. The hot-pressingconverted lithium metasilicate into lithium disilicate.

iii) The glass cylinders with nuclei (A) were subsequent to a firstcrystallisation according to ii) subjected to an additional thermaltreatment at 840 to 880° C. for 5 to 30 minutes. The analysis of thecrystal phases showed after this treatment a glass ceramic according tothe invention with lithium disilicate as main crystalline phase. Thecrystallised cylinders obtained after this second crystallisation weresubsequently processed by hot-pressing at a pressing temperature of900-950° C. using a pressing furnace EP600, Ivoclar Vivadent AG, todental restorations, e.g. inlays, onlays, veneers, partial crowns andcrowns.

(C) Machining (CAD/CAM Process) of Lithium Metasilicate

The rectangular glass blocks with nuclei (A) were subjected to a firstcrystallisation in accordance with (B) (ii) to effect crystallisation oflithium metasilicate. The produced lithium metasilicate glass blockswere then machined by CAD/CAM processes, e.g. Sirona, Cerec 2® or Cerec3®, to give dental restorations, e.g. inlays, onlays, veneers, partialcrowns and crowns. Subsequently, these restorations were subjected to asecond crystallisation at 840 to 880° C. for 5 minutes to 1 hour. Theanalysis of the crystal phases showed after this treatment a glassceramic according to the invention with lithium disilicate as maincrystalline phase.

TABLE I 1 2 3 4 5 6 7 8 9 SiO₂ 63.8 69.3 67.9 66.4 65.0 63.5 62.0 60.561.2 K₂O 3.0 3.8 3.7 3.6 3.5 3.4 3.4 3.3 0.8 Li₂O 13.6 14.4 14.1 13.813.5 13.2 12.9 12.6 13.0 Al₂O₃ 3.0 3.3 3.2 3.2 3.1 3.0 2.9 2.9 1.0 P₂O₅3.0 3.1 3.1 3.0 2.9 2.9 2.8 2.7 4.0 ZrO₂ 9.6 6.1 8.0 10.0 12.0 14.0 16.018.0 20.0 MoO₃ 4.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0100.00

TABLE II 10 11 12 13 14 15 16 17 18 19 20 SiO₂ 69.8 64.1 65.2 60.5 64.564.4 66.4 55.0 70.1 64.3 64.2 K₂O 2.0 5.0 3.5 3.3 2.0 2.0 4.0 3.6 3.01.0 Li₂O 16.0 13.3 12.0 15.0 13.4 13.5 13.6 15.0 9.0 13.2 13.2 Na₂O 0.1CaO 2.0 2.0 MgO 0.1 SrO 2.0 0.1 1.0 Al₂O₃ 0.2 5.0 3.1 2.9 2.0 2.0 3.04.0 3.5 2.9 0.5 La₂O₃ 6.5 Y₂O₃ 6.5 6.5 P₂O₅ 3.3 2.9 4.1 5.0 3.5 3.5 3.012.0 3.5 2.9 3.5 ZrO₂ 8.6 9.7 12.1 13.3 6.1 6.1 10.0 8.0 10.0 9.7 10.1Cs₂O 4.0 4.0 VO₂ 0.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0100.0 100.0 100.0

TABLE III 21 22 23 24 SiO₂ 61.3 62.6 64.9 64.9 K₂O 3.3 5.0 3.5 3.5 Li₂O12.7 12.7 13.5 13.5 CaO 3.0 Al₂O₃ 2.9 2.9 3.1 3.1 P₂O₅ 7.0 3.5 3.0 3.0ZrO₂ 9.0 11.3 10.4 10.9 F 0.5 MnO₂ 0.2 Fe₂O₃ 0.1 V₂O₅ 0.1 Tb₄O₇ 0.4 0.50.5 CeO2 1.3 1.0 0.6 Er₂O₃ 0.3 100.0 100.0 100.0 100.0

TABLE IV 25 26 27 28 SiO₂ 66.4 63.8 64.5 63.8 K₂O 3.0 3.2 3.0 Li₂O 13.613.6 13.8 13.6 Rb₂O 4.0 BaO 2.0 Al₂O₃ 3.0 3.0 3.0 3.0 Bi₂O₃ 4.0 P₂O₅ 3.03.0 3.5 3.0 ZrO₂ 10.0 9.6 10.0 9.6 WO₃ 4.0 100.0 100.0 100.0 100.0

TABLE V Crystal phases MP = main phase Glass Thermal treatment SP =secondary ceramic, no. (° C./min) or HT-XRD phase(s) 2 500/10, 650/20,MP: Li₂Si₂O₅ 840/7 SP: Li₃PO₄ Li₂SiO₃ 3 500/10, 650/20, MP: Li₂Si₂O₅840/7 SP: Li₃PO₄, Li₂SiO₃ 4 500/10, 650/20, MP: Li₂Si₂O₅ 840/7 SP:Li₃PO₄, Li₂SiO₃; Li₄SiO₄ 5 540/10, 690/20 MP: Li₂SiO₃ SP: none 5 540/10,650/20, MP: Li₂Si₂O₅ 840/7 SP: Li₂SiO₃; Li₄SiO₄ 6 540/10, 710/20 MP:Li₂SiO₃ SP: none 6 540/10, 650/20, MP: Li₂Si₂O₅ 840/7 SP: Li₄SiO₄ 7560/10 and HT-XRD MP: Li₂Si₂O₅ with cut-out SP: Li₃PO₄, Li₂SiO₃ at 860 8560/10 and HT-XRD MP: Li₂Si₂O₅, with cut-out SP: SiO₂, ZrO₂ at 900 9560/10 and HT-XRD MP: Li₂Si₂O₅, with cut-out SP: Li₃PO₄ at 920 14520/10, 650/20, 800/10 MP: Li₂Si₂O₅ SP: Li₃PO₄ 15 520/10, 650/20, 800/10MP: Li₂Si₂O₅ SP: Li₃PO₄, Li₂SiO₃ 16 520/10, 650/20, 800/10 MP: Li₂Si₂O₅SP: Li₃PO₄ 17 HT-XRD MP: Li₂Si₂O₅ with cut-out SP: Li₃PO₄ at 840 25520/10, 650/20, 800/10 MP: Li₂Si₂O₅ SP: Li₃PO₄ 26 520/10, 650/20, 850/10MP: Li₂Si₂O₅ SP: Li₃PO₄, Li₂SiO₃

In the HT-XRD analysis, a heating rate of approx. 2 K/min was used.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

The invention claimed is:
 1. A process for preparing a dentalrestoration comprising shaping of a lithium silicate glass ceramic or alithium silicate glass by (i) pressing or (ii) machining to the dentalrestoration, wherein the glass ceramic or the glass comprises 60.0 to71.0 wt.-% SiO₂ and 10.1 to 20.0 wt.-% ZrO₂.
 2. The process according toclaim 1, wherein the dental restoration is selected from inlays, onlays,crowns, veneers, facings or abutments.
 3. The process according to claim1, wherein the pressing is conducted at a temperature in the range offrom 700 to 1200° C.
 4. The process according to claim 1, wherein thepressing is conducted at a pressure in the range of from 2 to 10 bar. 5.The process according to claim 1, wherein the machining is carried outin a CAD/CAM process.
 6. The process according to claim 1, wherein theglass ceramic is selected from lithium metasilicate glass ceramic andlithium disilicate glass ceramic.
 7. The process according to claim 6,wherein the lithium disilicate glass ceramic is shaped by pressing tothe dental restoration.
 8. The process according to claim 6, wherein thelithium metasilicate glass ceramic or the lithium disilicate glassceramic is shaped by machining to the dental restoration.
 9. The processaccording to claim 1, wherein the glass comprises nuclei suitable forformation of lithium metasilicate and/or lithium disilicate crystals.10. The process according to claim 9, wherein the glass comprisingnuclei is shaped by pressing to the dental restoration.
 11. The processaccording to claim 1, wherein the glass is shaped by pressing to thedental restoration.
 12. The process according to claim 1, wherein afterthe shaping the glass ceramic or glass is heat treated to convert theglass ceramic or glass to lithium disilicate glass ceramic.
 13. Theprocess according to claim 1, wherein the glass or glass ceramiccomprises 10.4 to 20.0 wt.-% ZrO₂.
 14. The process according to claim 1,wherein the glass or glass ceramic comprises 60.0 to 69.0 wt.-% SiO₂.15. The process according to claim 1, wherein the glass or glass ceramiccomprises 9.0 to 17.0 wt.-% Li₂O.
 16. The process according to claim 1,wherein the glass or glass ceramic comprises 0.5 to 12.0 wt.-%nucleating agent, and wherein the nucleating agent is selected fromP₂O₅, TiO_(2,), Nb₂O₅ and/or metals.
 17. The process according to claim1, wherein the glass or glass ceramic comprises further alkali metaloxide in an amount of 1.0 to 7.0 wt.-%, and wherein the further alkalimetal oxide is selected from K₂O, Cs₂O and/or Rb₂O.
 18. The processaccording to claim 1, wherein the glass or glass ceramic comprises up to5.0 wt.-% alkaline earth metal oxide, and wherein the alkaline earthmetal oxide is selected from CaO, BaO, MgO and/or SrO.
 19. The processaccording to claim 1, wherein the glass or glass ceramic comprises 0.2to 10.0 wt.-% oxide of trivalent elements, and wherein the oxide oftrivalent elements is selected from Al₂O₃, Y₂O₃, La₂O₃, and/or Bi₂O₃.20. The process according to claim 1, wherein the glass or glass ceramiccomprises at least one further oxide of tetravalent elements, at leastone further oxide of pentavalent elements, and/or at least one oxide ofhexavalent elements.
 21. The process according to claim 1, wherein theglass or glass ceramic comprises at least one of the followingcomponents: Component wt.-% SiO₂ 60.0 to 69.0 Li₂O  9.0 to 17.0 K₂O  1.0to 5.0, Al₂O₃  0.5 to 5.0, P₂O₅  0.5 to 12.0, ZrO₂ 10.1 to 20.0.


22. The process according to claim 1, wherein a blank of the glass orglass ceramic is shaped to the dental restoration.
 23. Method of usinglithium silicate glass ceramic or lithium silicate glass for preparing adental restoration comprising shaping of the glass ceramic or the glassby pressing or machining to the dental restoration, wherein the glassceramic or the glass comprises 60.0 to 71.0 wt.-% SiO₂ and 10.1 to 20.0wt.-% ZrO₂.
 24. Method of using lithium silicate glass ceramic orlithium silicate glass according to claim 23, wherein the dentalrestorations are selected from inlays, onlays, crowns, veneers, facingsand abutments.